How could i place fire on top of every similar block in a large area? Bedrock

While I'm not aware of a command-based system that would achieve your objective the way you describe, it would certainly be possible to do this using entities and execute commands. The method I would use for this is below. I hope you find that it works for you and if you have any questions, please post a comment.

Method

1. First, put armour stands on top of the blocks you want the fire to appear on top of. Don't worry: You won't see them in the end result. I'm using end stone in my example but the block you use doesn't matter, since it's the armour stands that we're using to execute the commands (in fact, you could use the same block as the floor underneath your armour stands, making the fire trap totally invisible).
   
   


2. Next, you'll need to copy these commands into two command blocks. I'm activating them with a piston and a redstone block, but you can activate them however best suits your needs. I'll also explain how all of the commands in the method work at the end of my answer in case you don't know and want to but the commands you need are below.
   
   Fire on: /execute @e[type=armor_stand, name=firetrap] ~ ~ ~ setblock ~ ~ ~ fire
   Fire off: /execute @e[type=armor_stand, name=firetrap] ~ ~ ~ setblock ~ ~ ~ air
   
   


3. You may have noticed name=firetrap in the command. For the system to work, we need to name a name tag, 'firetrap' using an anvil and name all of the armour stands with it, so that they are all called 'firetrap'.



4. Finally, we need to do two things. We need to use the commands below to give our armour stands fire resistance so they don't burn and obviously invisibility.
   
   Fire resistance: /effect @e[type=armor_stand, name=firetrap] fire_resistance 100000 255 true
   Invisibility: /effect @e[type=armor_stand, name=firetrap] invisibility 100000 255 true

What the commands do

• /execute @e[type=armor_stand, name=firetrap] ~ ~ ~ setblock ~ ~ ~ fire
  Execute a command as all the armour stands within 100 blocks of this command block which are called 'firetrap'. Execute the command to set the block they are standing in as fire on behalf of them.


• /effect @e[type=armor_stand, name=firetrap] fire_resistance 100000 255 true
  Give all of the armour stands within 100 blocks which are called 'firetrap' fire resistance for 100000 seconds (normally it would be this number of seconds but the number 100000 means indefinitely) and with 255 intensity. Set hide particles to true.

Result